Surface treatment for polystyrene which is to be electroless plated and compositions therefor



United States Patent 3 472,678 SURFACE TREATMENT FOR POLYSTYRENE WHICHIS TO BE ELECTROLESS PLATED AND COMPOSITIONS THEREFOR Paul F. Bruins,Douglastown, Isaac Benghiat, New City, and Harold R. Frost, New York,N.Y., assignors to Hexagon Laboratories, Incorporated, Bronx, N.Y., acorporation of New York No Drawing. Filed May 3, 1967, Ser. No. 635,675Int. Cl. B44d 1/38, 1/40 U.S. Cl. 117-47 5 Claims ABSTRACT OF THEDISCLOSURE Treatment of polystyrene with a :mixture of 50;, complexedwith trimethyl phosphate and a halogenated aromatic hydrocarbon prior todipping in concentrated acid and electroless plating metal on thesurface of polystyrene, and treatment bath compositions comprising 80:,complexed with trimethyl phosphate, and a halogenated aromatichydrocarbon.

The present invention is directed to a treatment of the surface ofpolystyrene which is to be electroless plated and compositions therefor.

In recent years there has been a rapid growth of the plating of plasticswith metals. Thus, plated plastics offer a number of advantages overmetals including lightweight, corrosion resistance, greater designlatitude, simplified assembly, controlled conductivity and economy.

It has long proven to be desirable to plate polystyrene and, inparticular, high impact polystyrene. By high impact" polystyrene ismeant rubber modified polystyrene, a wide variety of which arecommercially available. Barrel plating of polystyrene, and high impactpolystyrene, has been done for many years. In this method, a completeenvelope of electroplated metal surrounds the molded object. Theelectroplating baths used in barrel plating do not produce a smooth,bright deposit, but this plated metal can readily be polished by barrelpolishing. There is no need for a strong bond between the plated metaland the polystyrene substrate. Recently, however, electroplating bathshave been developed for the bright plating of plastics. These baths makepossible plating of large, irregular shaped moldings which cannot besuccessfully barrel polished, and which cannot be economically polishedby buffing. In this case, a strong bond between the plated metal and theplastic substrate is essential. Methods of pre-treating the surface ofplastics such as ABS (acrylonitrile-butadiene-styrene copolymers) havebeen developed. The treating bath consists of a concentrated aqueoussolution of chromic acid. This oxidizes the surface of the ABS plastic,so that a strong bond is developed between the surface and the platedmetal. However, the plating of polystyrene or high impact polystyrene bythis method has not proved to be commercially successful. The chemicalinertness of the polystyrene and high impact polystyrene is probably thecause of the lack of chemical attack. Thus, prior attempts to brightplate polystyrene or high impact polystyrene, has resulted in a low bondstrength between the surface and the plated metal.

In the past, silver has been the most successful electroless metaldeposit on the surface of plastics in order to make them conductive andreceptive to thicker deposits of electroplated metal. The high cost andscarcity of silver has resulted in the development of electroless copperand electroless nickel plating baths which have been successfully usedfor making the surface of treated ABS moldings receptive toelectroplated copper or nickel deposits. Treatment of polystyrene andhigh impact poly- 3,472,678 Patented Oct. 14, 1969 styrene surfaces bysimilar methods has not resulted in producing a receptive surface forelectroplating.

This invention has as an object the provision of a surface treatment forpolystyrene, and in particular high impact polystyrene, whereby thesurface may be conditioned to receive a satisfactory copper coating byelectroless plating.

This invention has as another object the provision of a surfacetreatment bath for conditioning polystyrene and in particular, highimpact polystyrene, for electroless plating of copper or nickel inpreparation for subsequent electroplating.

Other objects will appear hereinafter.

The present invention is directed to the electroless plating ofpolystyrene, and in particular high impact polystyrene. The presentinvention is broadly applicable to a wide variety of polystyrene whichare available. The properties of both the general purpose polystyrenesand the impact polystyrenes comprehended within the subject inventionare generally set forth in the Table I at pages 308 and 309 of theModern Plastics Encyclopedia, 1967, published by McGraw-Hill, Inc. Wehave found that the surface treatment and the treating bath of thepresent invention may be used with all types of polystyrene. We havedetermined that there is an improvement in the adhesion between thepolystyrene and the plated metal with increasing molding temperature,and hence it is normally desirable that as high a molding temperature asis feasible (temperature degradation is to be avoided), should be used,prior to the preplating treatment. As with all plastic plating methods,the polystyrene to be plated in accordance with the present inventionshould be as clean as is feasible. To this end, mold release agentsshould be avoided, and rigorous care taken to achieve clean and carefulhandling of the polystyrene which is to be plated.

The surface treating baths of the present invention comprise a complexof from 20 to 65 parts by weight of sulfur trioxide and to 35 parts byweight of trimethyl phosphate, and 2 to 20 parts by weight of ahalogenated aromatic hydrocarbon solvent, which is substantially stableto the aforesaid complex.

We have found that when the complex contains more than about 65 parts byweight of sulfur trioxide to 35 parts by weight of the trimethylphosphate the shelf life of the preplating bath decreases drastically.While the preferred concentration ratio of sulfur trioxide to trimethylphosphate will vary depending upon the type of polystyrene which isused, we have found that in most cases, and in particular with highimpact polystyrene, a concentration ratio of from 40 to 55 parts byweight of sulfur trioxide to 60 to 45 parts by weight of trimethylphosphate is to be preferred.

The presence of the halogenated aromatic hydrocarbon solvent, which isstable in the aforesaid complex, in a concentration of above about 2weight percent based on the total mixture enhances the efficacy of thepreplating bath. In particular, we have achieved excellent results witho-dichlorobenzene.

The optimum concentration of the halogenated aromatic hydrocarbonsolvent is dependent upon three factors which are within the control ofthe user of the process of the present invention, namely the type ofpolystyrene to which the invention is applied, the time duration of thesurface treatment, and the temperature of the bath.

The concentration of the holegenated aromatic hydrocarbon solvent shouldbe sufficiently low so that no undue solvent attack of the plastic, orundue roughening of the plastics surface will result from the preplatingtreatment.

Examples of halogenated aromatic hydrocarbons which may be used in thepresent invention include orthodichlorobenzene, metadichlorobenzene,metadibromobenzene,

fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene.

In the process of the present invention the polystyene, which has beenthoroughly cleaned, is treated in the bath at a temperature between 25C. and below a temperature which will result in heat distortion for theparticular polystyrene and molded shape which is being treated. As ageneral rule, the maximum temperature which can be used without adverseheat distortion should be used so as to reduce the treatment time, andto minimize the roughening of the surface. With most polystyrenes thiswill range from about 60 to 80 C. There is a treatment temperature-timeinterrelationship, with the higher the temperature the shorter thetreatment time. The optimum conditions will vary depending up the natureof the plastic, and the chemical composition of the bath. However, theseconditions can be determined by one having ordinary skill in the art byroutine testing of a non-inventive nature. Generally, the longer thetreatment, the stronger the bond strength of the plated copper to thepolystyrene. However, this is offset by the increased roughness of thetreated surface, so that in many cases there is a balance betweenduration of treatment, desired bond strength, and appearance of theplated surface.

After the surface treatment in the treatment bath it is necessary toremove the residues left on the plastic surface. This is accomplished bythorough water rinsing, followed by soaking at 25 C. or higher, in aconcentrated aqueous acid solution. The acids that work well for thisfinal treatment are concentrated hydrochloric acid, phosphoric acid, ornitric acid. The preferred acid is hydrochloric acid, and soaking timesrange from minutes to 60 minutes, the time varying inversely with thetemperature. Hydrochloric acid is the preferred acid, because it is mosteasily rinsed off after the soaking, and small traces of it do notcontaminate the subsequent sensitizing and activating baths.

The electroless plating of plastics is usually preceded by sensitizingand activation steps, well known in the art. In the sensitizing step theplastic is immersed in an acidified solution of stannous chloride, andin the activation step the surface is treated with a solution ofacidified palladium chloride. This is immediately followed byelectroless plating which may be a well known formula or a proprietarybath containing copper, nickel, silver, or gold.

While copper is most generally used as the metal which is electrolessplated, the process of the present invention is equally applicable tothe electroless plating of other metals, such as nickel, silver, orgold. The conditions for electroless plating and the apparatus and meansused form no part of the present invention, and come within the skill ofone having ordinary skill in the electroless plating art. A largeliterature has developed in connection with the plating of plastics, seethe article on Electroplating Plastics at pages 1019 through 1024 ofModern Plastics Encyclopedia, 1967, and the references cited therein.

The thickness of the electroless deposited copper (or other metal) willvary depending upon the requirements of the user, and the means forregulating such thickness can be accomplished by techniques available toone having ordinary skill in the art. With copper the thickness of theinitial electroless plating may be on the order of from ten millionthsof an inch thickness up to as much as several mils. The ultimatethickness of the plating depends upon the end use of the finishedproduct, and the electroless plating may be followed by conventionalelectroplating on the original electroless plating to any desiredthickness. The plating subsequent to the electroless plating may beaccomplished by conventional means, including the use of the new brightplating baths.

In order to illustrate the present invention, Lustrex HT88, a highimpact polystyrene containing 88% by weight of styrene, and 12% ofbutadiene, and manufactured by the Monsanto Chemical Company, was usedas the polystyrene being treated. The bond strength test in which theplated metal was pulled at 90 to the treated plastic surface, and theforce in pounds per inch measured, is peel strength.

In each of the following examples the treatment bath contained 46.2%sulfur trioxide in trimethyl phosphate, and the final acid conditionerwas concentrated hydrochloric acid.

All test samples were then sensitized with a solution containing 10grams of stannous chloride and 40 ml. of concentrated hydrochloric acidper 1000 ml. of water. The sensitization was accomplished by treatmentof the samples with-this solution for 2 minutes at room temperature. Thesamples were then activated by dipping them for two minutes at roomtemperature into a solution containing 1 gram of palladium chloride and10 ml. of concentrated hydrochloric acid per gallon of water. All of thetest samples were then plated with an electroless copper plating bathwhich plated at the rate of .1 of a mil per hour, and they were platedfor 10 minutes in this bath, thus producing an electroless copper plate0.0166 mils thick. This was followed by electroplating bright copper toa thickness of 3 mils in order to provide sufficient thickness for thepeel test.

EXAMPLE 1 The sample was treated in the sulfur dioxide bath containing10% orthodichlorobenzene at a temperature of 25 C. for 30 minutes, andwas soaked in concentrated hydrochloric acid at 25 C. for 60 minutes.The peel strength obtained was 2 pounds per inch. The bright platedsurface reflected some roughness from the etched surface.

EXAMPLE 2 The sample was treated in the sulfur trioxide bath containing3 /2% orthodichlorobenzene at 60 C. for 60 minutes, followed by 60minutes soak at 25 C. in the concentrated hydrochloric acid bath. Theresulting peel strength was 3 pounds per inch, and the plated surfacewas bright and smooth.

EXAMPLE 3 The sample was treated in the sulfur trioxide bath containing7% orthodichlorobenzene at 60 C. for 7 minutes, followed by a soak for60 minutes in concentrated hydrochloric acid. The resulting peelstrenght was 3 /2 pounds per inch, and the plated surface was smooth andbright.

EXAMPLE 4 The same procedure was used as in Example 3, except the timeof treatment in the sulfur trioxide bath was increased to 10 minutes.The peel strength increased to 7 pounds per inch, and the plated surfacewas moderately roughened by the treatment.

EXAMPLE 5 The same procedure was used as in Example 3, except the timein the sulfur trioxide bath was increased to 13 minutes. The resultingpeel strength increased to 9 pounds per inch, and the plated surface wasroughened by the treatment.

EXAMPLE 6 The same procedure was used as in Example 3, but the time inthe sulfur trioxide bath was increased to 30 minutes. The result was apeel strength of 14 pounds per inch, but the surface was very rough.

EXAMPLE 7 The sample was treated in the sulfur trioxide bath containing7% orthodichlorobenzene at 60 C. for 10 minutes, and was followed by a20-minute soak in the concentrated HCl. The resulting peel strength was6 /2 pounds per inch, and the plated surface was moderately rough.

EXAMPLE 8 A sample of general purpose polystyrene was treated in thesulfur trioxide bath containing 7% orthodichlorobenzene for 12 minutesat 60 C., and followed by a soak in concentrated hydrochloric acid for12 minutes at 25 C. The resulting peel strength was 5 pounds per inch,and the plated surface was moderately rough.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

In the following claims polystyrene" includes both high impactpolystyrenes which are copolymers of polystyrene and a synthetic rubber,as well as general purpose polystyrene.

It is claimed:

1. A treating bath for the surface conditioning of polystyrene which isto be electroless plated consisting essentially of a solution of acomplex of from 20 to 65 parts by weight of sulfur trioxide and from 80to 35 parts by weight of a trimethyl phosphate, and a halogenatedaromatic hydrocarbon solvent which is stable in the presence of saidcomplex, the concentration of said halogenated aromatic hydrocarbonsolvent being sufficient to enhance the preplating efficacy of the bathfor polystyrene and below the concentration which adversely roughens thesurface of the polystyrene or adversely swells the polystyrene.

2. A treating bath in accordance with claim 1 in which the halogenatedaromatic hydrocarbon solvent is orthodichlorobenzene.

3, A treating bath in accordance with claim 1 in which the halogenatedhydrocarbon solvent is present to the extent of 2 weight percent andbelow a concentration of 20 Weight percent.

4. A process for preconditioning and plating polystyrene which comprisescontacting the polystyrene with a preplating bath of the type set forthin claim 1 at a temperature of between 30 C. and below a temperaturewhich will result in adverse heat distortion of said polystyrene for atime sufficient to condition the surface of the polystyrene forelectroless plating, rinsing the polystyrene with water to remove thepreplating bath, treating the surface of the polystyrene with a strongacid selected from the group consisting of hydrochloric acid, phosphoricacid, and nitric acid, and the electrolessly metal plating the treatedpolystyrene.

5. A process in accordance with claim 4 in which the acid ishydrochloric acid.

References Cited UNITED STATES PATENTS 3,035,944 5/1962 Sher 117473,222,218 12/1965 Beltzer et a1. 11747 3,305,460 2/1967 Lacy 117473,379,556 4/1968 Chiecchi 1l7-47 MURRAY KATZ, Primary Examiner U.S. Cl.X.R.

